Rule-based technique to automatically determine the final scan gain in storage phosphor radiography

Abstract

In storage phosphor radiography, a portion of the x-ray energy transmitted through the subject is absorbed by a storage phosphor plate. When stimulated by visible light of the right wavelength, the phosphor plate emits light (at another wavelength) in proportion to the absorbed energy. To obtain the radiographic image, the plate can be scanned in a raster fashion and the emitted light can be photoelectrically detected. The detected signal is then amplified, digitized, processed and finally printed on film, or displayed on a cathode ray tube (CRT). Setting the amplification gain of this scanning process, the so-called final scan (or final read-out) gain, is of interest in maximizing the information extracted from the phosphor. One method of setting the final scan gain invokes the use of a preliminary read-out (pre-scan). A pre-scan can be conducted on the phosphor plate by use of a stimulating ray having a stimulating energy lower than the stimulating energy in the final scan. The present invention provides a technique that uses the pre-scan image histogram to automatically determine the final scan gain in storage phosphor radiography.

We claim: 
     
       1. A method of adjusting final scan gain to produce a final image in a storage phosphor radiography system, comprising the steps of: preliminarily reading out an exposed storage phosphor by stimulating the storage phosphor with a lower intensity stimulation to produce a preliminary image signal, obtaining a histogram of the preliminary image signal, finally reading out said exposed storage phosphor by stimulating said storage phosphor with a higher intensity stimulation based on the histogram to produce a final image signal, and in a digital computer, a. generating a histogram from the results of the preliminary read-out;   b. detecting peaks and/or clusters of peaks A 1  and A 2  in the histogram;   c. determining one major set of peaks or clusters, A, representing structures of interest; and   d. determining a gain factor g that will locate the major set of peaks at desired gray level locations in the final image based on a set of exam dependent rules and image recording conditions.   
     
     
       2. The method claimed in claim 1, wherein said finally reading out step includes using a laser for conducting final scan stimulation of the storage phosphor, detecting a final image signal with a photomultiplier tube, and amplifying the final image signal by means of a variable gain amplifier, and further comprising the step of: employing system parameters including pre-scan gain, laser power, photomultiplier tube setting, and the gain factor g to compute an electronic gain factor g f  which is applied to the variable gain amplifier.   
     
     
       3. The method claimed in claim 1, wherein the steps of detecting peaks and/or clusters of peaks comprises the steps of: a. forming a cumulative distribution function (CDF) from the histogram;   b. smoothing the cumulative distribution function with a sliding window average of size N 1 , to produce a smoothed cumulative distribution function;   c. subtracting the smoothed cumulative distribution function from the cumulative distribution function to produce a peak detection function;   d. employing the peak detection function to locate a set of peaks, A 1  in the histogram by 1). identifying positive to negative zero crossings of the peak detection function as the start of a detected peak and   2). identifying a maximum following such a positive to negative zero crossing as the end of a detected histogram peak; and     e. repeating steps a-c with a window of size N 2  where N 1  >N 2  to locate a second set of peaks A 2 .   
     
     
       4. The method claimed in claim 3 wherein the step of determining one major set of peaks, A, is performed according to the following rules: R1. an A 1  -peak qualifies for the set A if (i) it is an independent peak, or   (ii) it is not an independent peak but the total number of the significant overlaps is less than t, where t is an empirically predetermined parameter. (If an A 2  peak overlaps significantly with an A 1  -peak then the overlap is said to be a `major overlap` if the ratio of the number of pixels contained in the overlap to the total number of pixels contained in the A 1  -peak exceeds the value R maj .); and     R2. an A 2  -peak qualifies for the set A if (i) it is an independent peak, or   (ii) it is not independent, but its overlap with the A 1  -peak is a major overlap and the total number of A 2  -peaks that have major overlaps with the A 1  peak is at least t, or   (iii) it is not independent and its overlap with the A 1  -peak is not a major one, but there exist at least t other A 2  -peaks with major overlaps with that A 1  -peak, in which case, adjacent peaks that do not have major overlaps with the A 1  -peak are combined into single peaks.     
     
     
       5. The method claimed in claim 1, wherein the sets of exam dependent rules include a set of rules for chest exams comprising the following rules: a. IF the histogram is unimodal, THEN the gain is set such that e 1  is mapped to gray level (or code value) Q c   u  : g=Q c   u  /e 1  wherein, the superscript denotes the modality of the histogram and the subscript denotes the exam category of chest exam;   b. IF the histogram is not unimodal, THEN (1). IF at least a predetermined percentage, P2%, of the total number of pixels attain values in the interval [e 2 , q max  -1] (where q max  denotes the largest gray level present in the pre-scan image), THEN the gain is determined by the following rule: First, the local maximum of the histogram in the interval [s 2 , e 2  ] is determined, let m 2  denote the code value at which the local maximum occurs, then the interval [m 2 , q max  -1] is searched for the smallest code value at which the histogram attains a value less than or equal to K1h(m 2 ) (K1<1 is a predetermined coefficient),   (2). IF the percentage of pixels that attain values in the interval [e 2 , q max  -1] is greater than or equal to a predetermined percentage, P1, but smaller than P2 THEN the previous rule is used with K2h(m 2 ) (K1<K2<1 is a predetermined coefficient),   (3). IF the percentage of pixels that attain values in the interval [e 2 , q max  -1] is less than P1 THEN (a). IF the x-rays are not collimated i. IF the histogram is bimodal, THEN       ((1)). IF the percentage of the total number of pixels that attain the value q max  is less than or equal to P c  %, THEN, ((a)). IF the slope of the CDF between e 1  and s 2  is greater than the predetermined threshold S c , THEN the gain is set such that s 2  is mapped to code value Q c  : g=Q c  /s 2 ,   ((b)). IF the slope of the CDF computed between e 1  and s 2  is less than or equal to the predetermined threshold S c , THEN the gain is set such that a code value between e 1  and s 2 , determined from a convex combination of e 1  and s 2 , i.e., (L c )e 1  +(1-L c )s 2 , is mapped to code value Q c ,     ((2)). IF the percentage of the total number of pixels that attain the value q max  is greater than P c  %, THEN the gain is set such that e 2  is mapped to code value Q c  : g=Q c  /e 2 , ii. IF the histogram is not bimodal, THEN the gain is set such that e 2  is mapped to code value Q c  : g=Q c  /e 2 , (b). IF the x-rays are collimated, THEN the gain is set as in b.(1).         
     
     
       6. The method claimed in claim 1, wherein the sets of exam dependent rules include a set of rules for extremity exams comprising the following rules: a. IF the histogram is unimodal, THEN, 1). IF the x-rays are not collimated, THEN the gain is set as g=Q e   u  /e 1  provided that not more than P e  % of the pixels are mapped to the maximum code value of the system (e.g. 4095 in a 12-bits/pixel system) in the final output, ELSE the gain is set such that g=Q e   u  /e where e (e<e 1 ) is determined such that 1% of the pixels are mapped to the maximum code value of the system in the final output, where the superscript denotes the modality of the histogram and the subscript denotes the exam category of extremity,   2). IF the x-rays are collimated, THEN the gain is set such that e 1  is mapped to code value Q e   u ,c ; g=Q e   u ,c /e 1 , where the superscript c reflects the fact that the x-rays are collimated,     b. IF the histogram is not unimodal, THEN 1). IF the x-rays are not collimated, (a). IF the histogram is bimodal, THEN i. IF the slope of the CDF computed between e 1  and s 2  is greater than the predetermined threshold S 3 , THEN the gain is set as g=Q e  /e 2  provided that not more than P e  % of the pixels are mapped to the maximum code value of the system (e.g. 4095 in a 12-bits/pixel system) in the final output, ELSE the gain is set such that g=Q e  /e where e (e<e 2 ) is determined such that 1% of the pixels are mapped to the maximum code value of the system in the final output,   ii. IF the slope of the CDF computed between e 1  and s 2  is less than or equal to the predetermined threshold S e , THEN the gain is set such that a code value between e 1  and s 2 , determined from a convex combination of e 1  and s 2 , i.e., (L e )e 1  +(1-L e )s 2 , is mapped to code value Q e ,     (b). IF the histogram has more than two clusters, THEN i. IF the slope of the CDF computed between e 2  and s 3  is greater than the predetermined threshold S e , THEN the gain is set as g=Q e  /e 3  provided that not more than P e  % of the pixels lie in [e 3 ,q max  -1], ELSE the gain is set such that the percentage of the pixels that are mapped to the maximum code value of the system in the final output is 1%,   ii. IF the slope of the CDF computed between e 2  and s 3  is less than or equal to the predetermined threshold S e , THEN     ((a)). IF the slope of the CDF computed between e 1  and s 2  is greater than the predetermined threshold S e , THEN the gain is set such that a code value between e 2  and s 3 , determined from a convex combination of e 2  and s 3 , i.e., (L e )e 2  +(1-L e )s 3 , is mapped to code value Q e ,   ((b)). IF the slope of the CDF computed between e 1  and s 2  is less than or equal to the predetermined threshold S e , and the second cluster is closer to the third, THEN     ((1)). IF the histogram has three clusters, THEN the gain is set such that a code value between e 1  and s 2 , determined from a convex combination of e 1  and s 2 , i.e., (L e )e 1  +(1-L e )s 2 , is mapped to code value Q e ,   ((2)). IF the histogram has four clusters, THEN the gain is set such that s 3  is mapped to Q e  : g=Q e  /s 3 ,   ((3)). IF the histogram has more than four cluster, THEN the gain is set such that e 3  is mapped to Q e  : g=Q e  /e 3 , ((c)). IF the slope of the CDF computed between e 1  and s 2  is less than or equal to the predetermined threshold S e , but the second cluster is closer to the first, THEN the gain is set such that a code value between e 2  and s 3 , determined from a convex combination of e 2  and s 3 , (L e )e 2  +(1-L e )s 3 , is mapped to code value Q e ,     2). IF the x-rays are collimated, THEN the gain is set such that the end point of the last cluster is mapped to code value Q e   c , where the superscript c reflects the fact that the x-rays are collimated.     
     
     
       7. The method claimed in claim 1, wherein the sets of exam dependent rules include a set of rules for abdomen exams, comprising the following rules: a. IF the histogram is unimodal, THEN 1). The gain is set as g=Q a   u  /e 1  provided that not more than P a  % and not less than 0.5% of the pixels are mapped to the maximum code value of the system in the final output, ELSE the gain is set such that g=Q a   u  /e where e is determined such that 0.5% of the pixels are mapped to the maximum code value of the system in the final output, where the superscript denotes the modality of the histogram, and the subscript denotes the exam category abdomen, and     b. IF the histogram is not unimodal, THEN 1). IF the x-rays are not collimated, (a). IF the histogram is bimodal, THEN i. IF the slope of the CDF computed between e 1  and s 2  is greater than the predetermined threshold S a , THEN the gain is set as g=Q a   u  /e 2  provided that not more than P a  % of the pixels are mapped to the maximum code value of the system (e.g. 4095 in a 12-bits/pixel system) in the final output, ELSE the gain is set such that g=Q a   u  /e where e (e<e 2 ) is determined such that 0.5% of the pixels are mapped to the maximum code value of the system in the final output,   ii. IF the slope of the CDF computed between e 1  and s 2  is less than or equal to the predetermined threshold S a , THEN the gain is set such that a code value between e 1  and s 2 , determined from a convex combination of e 1  and s 2 , i.e., (L a )e 1  +(1-L a )s 2 , is mapped to code value Q a ,     (b). IF the histogram has more than two clusters, THEN i. IF the slope of the CDF computed between e 2  and s 3  is greater than the predetermined threshold S a , THEN the gain is set as g=Q a   u  /e 3  provided that not more than P a  % of the pixels lie in [e 3 , q max  -1], ELSE the gain in set such that the percentage of the pixels that are mapped to the maximum code value of the system in the final output is 0.5%,   ii. IF the slope of the CDF computed between e 2  and s 2  is greater than equal to the predetermined threshold S a , THEN,     ((a)). IF the slope of the CDF computed between e 1  and s 2  is greater than the predetermined threshold S a , THEN the gain is set such that a code value between e 2  and s 3 , determined from a convex combination of e 2  and s 3 , i.e., (L a )e 2  +(1-L a )s 3 , is mapped to code value Q a ,   ((b)). IF the slope of the CDF computed between e 1  and s 2  is less than or equal to the predetermined threshold S a , and the second cluster is closer to the third, THEN the gain is set such that a code value between e 1  and s 2 , determined from a convex combination of e 1  and s 2 , i.e., (L a )e 1  +(1-L a )s 2 , is mapped to code value Q a ,   ((c)). IF the slope of the CDF computed between e 2  and e 3  is less than or equal to the predetermined threshold S a , but the second cluster is closer to first, THEN the gain is set as g=Q a  /e 3 ,     2). IF the x-rays are collimated, THEN the gain is set such that the end point of the last cluster is mapped to code value Q a   c , where the superscript c reflects the fact that the x-rays are collimated.     
     
     
       8. The method claimed in claim 1, wherein the storage phosphor radiography system includes a quality control station having a display monitor, further comprising the steps of: a. scaling the preliminary-scan image pixel-by-pixel to produce a quality control image, and   b. displaying the quality control image on the monitor prior to final read out of the image.   
     
     
       9. A method of adjusting final scan gain to produce a final image in a storage phosphor radiography system, comprising the steps of: preliminarily reading out a storage phosphor by stimulating the storage phosphor with a lower intensity stimulation to produce a preliminary image signal, obtaining a histogram of the preliminary image signal, finally reading out said storage phosphor by stimulating said storage phosphor with a higher intensity stimulation based on the histogram to produce a final image signal, and comprising the steps of: a. computing a gain factor g for the final scan stimulation, and   b. scaling a preliminary scan image pixel-by-pixel to produce a quality control image signal, and   c. displaying the quality control image signal on a monitor prior to conducting the final scan stimulation, whereby an operator viewing the quality control image may order a re-take of the image prior to final read-out in the event that the quality control image is unsatisfactory.